Duskmon's a little different from the rest of the enemies in Frontier. He is Kouji's twin brother Kouichi *CUTENESS ALERT!!! XD*, spirit-evolved with the corrupted Human Spirit of Darkness. When Kouichi missed the train to the Digital World, Cherubimon somehow got his spirit into the Digital World. He was given the corrupted Spirit and when he used it to spirit-evolve, he was totally brainwashed. He only had one mission - to destroy the other Digidestined, including his brother. He's not quite like the other evil hybrids because he doesn't follow Cherubimon of his own accord. In fact, he has moments where he declares that he follows only himself.
Eventually, Duskmon / Kouichi remembers that Kouji's his brother, his Spirits are purified and he evolves into Lowemon - the true Warrior of Darkness.